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A very long shot, but Rick Riordan's The Kane Chronicles series? Can we get a better timeframe for when your friend read this?
Afraid not. He says it was some time between 2004 and 2007.
The Egypt Game?
No, I'm afraid that's not it either.
A few series come to mind when I read this, so to help rule some of them out, I need to know the answers to the following: Does your friend remember, exactly, how the protagonists got to Egypt? Did they just open the basement door and see the pyramids immediately, or was there more to it? Also, did they usually go of their own accord or were they yanked from the present without warning? Were their parents involved or, at the very least, aware of everything? Was Narnia Time in effect or was San Dimas Time used?
This is for a friend of mine, so here's his description:
"I read a book when I was younger, and I cannot remember the title or the author at all. If I’m right, there was a brother and sister, and they lived somewhere in England, I think (Cornwall?). It was rainy and they would visit (I think?!) ancient Egypt through their basement, and there were a few books based on this."
Edited by AlxxlA